STEEL HULLS with Composite Superstructure / Topsides

Discussion in 'Boatbuilding' started by brian eiland, Jun 16, 2013.

  1. brian eiland
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    brian eiland Senior Member

    Pretty tough stuff, .....even when it was mixed incorrectly??
     
  2. groper
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    groper Senior Member

  3. waikikin
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    waikikin Senior Member

  4. MikeJohns
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    MikeJohns Senior Member

  5. MikeJohns
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    MikeJohns Senior Member

    Often on smaller craft they are just flat bar, as the vessel gets bigger they usually L or T and the skin is a significant integral part of the beam ( monocoque).

    Yes rusting behind the longitudinals is really not an issue these days. They are also quite quick to set up on a larger sized craft such as you are considering since they slide into slots in the transverse frames and the radius of curvature is not great if they are sensibly run. They are also good for pulling the plate in to fit in some parts of the hull especially if being built from flat sheet as opposed to being rolled.

    I said before that structural members can be reduced in size if they have full end fixity. That is if the longitudinal is well fixed at the ends or is continuous then the size of it can be reduced to between ¼ and 1/8 depending on whether we consider a point load or a distributed panel load. If it’s continuous it ‘shifts’ a lot of the stress into the adjacent structural member that goes into a reverse loading across the other side of the support. There’s a neutral stress point of contraflexure that’s worth trying to understand and the result is a much lower stress and a much lighter structure for the same strength.

    End fixity is achieved either by having a continuous structural member or by bracketing it’s termination onto a more rigid and stronger part of the structure. For example to get end fixity of intermediate GRP longitudinals you’d have to incorporate a complex attachment at each end. But if you didn’t do this your longitudinals would be around 10 times heavier and I’d estimate an order of magnitude more expensive not counting the time to shape clamp and glue.

    A small longitudinal ( steel rather than GRP)has another big advantage in that the neutral axis of the resultant beam runs though very close to the weld attachments. So the welds experience very minimal stress which is limited to the extreme fibres. A large GRP structural element will have its neutral axis well off the plate and consequently the glue line will have a much higher stress than the welds would have experienced.

    Any collision inducing a stress past yield ( a dent) would result in a very quick failure of the bonded substructure and or the GRP itself and again the inherent values of the great strength if an isotropic monocoque type construction have been lost. Ie the great energy absorption ability of a monocoque structure.

    I think you will have added substantial cost weight and construction time. And we also need to consider the curved sections the longitudinals usually describe for much of the vessel . You are not going to easily bend or shape short length of stock I beam to glue them . And to carry the FRP members continuoulsly is not sensibly viable when its so easy to use metal.

    But also note that you can actually design without longitudinals! Most of the Dutch boats use transverse only framing, but the framing is a bit closer and we sometimes need to use a side girders or commonly incorporate a hard chine as a girder. I prefer longitudinals though because the number of transverses are approx doubled in this method and there’s less work and a stronger result running longitudinals..

    Nylons way way too elastic for structural elements reinforced or not. It’s great for rollers and bearings rubbing strips and ropes, its incredibly tough. But you could never use nylon for anything structural.

    I think they have been considered, composite structures are old hat and structural courses go into this in some detail because there is a lot to be considered at quite a detailed level.

    Note that gluing wooden boats works well because the glue properties work well with the wood properties. But steel is an order of magnitude stronger and the glue is a lot weaker and steel is so easy and cheap to weld and steel is a gift from god so to speak when it comes to structural integrity of welds, fatigue response and incredible ductility to failure.

    Also the bonding is another issue altogether, successful bonding requires close mating surfaces and strong clamping forces and minimal amounts of glue. The glue bond can never be stronger than the weakest surface, so in this case it’s the properties of the polymer that makes up the matrix of your FRP or the Polyster.


    I really think it’s a shame to compromise the very real advantages of full metal construction.

    I haven’t covered stresses from differential thermal expansion either on which we could go into some detail.
     
  6. pdwiley
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    pdwiley Senior Member

    Umm.

    You have quite obviously never built a steel framed boat.

    You have equally obviously never read any of the books on steel boat building, or if you have, you didn't understand what you read.

    Longitudinal stringers are NOT fully welded to the hull plate in almost all cases. The only time I personally know this was done was for an aluminium workboat we had built for Antarctic service, and we didn't want any chance that ice could form between the stringers & plate. It was a costly PITA and almost certainly unnecessary.

    Otherwise you stitch weld as per the designer's specification.

    So it's not worth addressing the other stuff you wrote until you deal with your misconception about basic construction.

    FWIW my 12m hull has 4mm plate and 32x6 FB stringers on 300mm spacing from the CL to the chine and 300mm spacing from the deck edge down. Frames are 65x6 FB on variable spacing as per design.

    PDW
     
  7. Ace Dragon
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    Ace Dragon Polyurea and Spray Foam

    If polyurea is not tested - sure a lot of happy customers

    Lots more job profiles on SPI and Versaflex web sites.
     
  8. brian eiland
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    brian eiland Senior Member

    Adhesive Bonding of Bulkheads

    (Mike I haven't forgotten you. ;) I will return with a reply to your last posting, but I got sidetracked with some other work, plus preparations to leave tomorrow for a month long visit to Thailand)

    Meanwhile in my organizing mode I found this...
    ...a reference from EuropeanBoatbuilder...

    Check out the strength and ductility of this adhesive product.
     

    Attached Files:

  9. brian eiland
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    brian eiland Senior Member

    Steel Stringer Questions

    A question I wish to clarify about your 'flat bar' stringer. This stringer has a certain thickness associated with it, correct. When you are welding it in I assume you are talking about stitch welding at intervals rather than continuous welding the whole length. My questions:
    1) generally what sort of pattern might this stitching be, ie, welded for 1 foot, skip 1 foot, welded another foot, etc ?

    2) Depending on the thickness of this flat bar stringer is there a requirement to weld both sides of it to the hull's skin at each 'stitch', OR just weld one side?

    If only one side of this juncture is welded, doesn't that present some extra problems with later preparations to prevent corrosion in these nooks & cranny's between the underside of the flat bar and the hull's skin??

    3) Am I correct that this 'stitch welding' of the stringer down one side of the hull entity should be complimented with the welding of its opposite member on the other side of the hull??...jumping from one side of the hull to the other to prevent any warping??
     
  10. brian eiland
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    brian eiland Senior Member

    Sheer Clamp questions

    I assume this is term we can apply to refer to the L-channel piece of steel that could be welded down the length of the inner skin of hull along the sheer to support the edge of the deck at this deck-to-hull joint. One side face of this channel would be welded to the hull skin, while the other would provide a 'shelf' for the edge of the deck. I assume BOTH edges of this flat surface of the channel would need to be welded to the hull's skin?

    What happens to that air space that is left between the flat surfaces of the hull's skin and the channel face??

    I imagine like the stringers above, this sheer clamp might also only be 'stitch welded' at intervals? Would this further aggregate potential corrosion problems with this 'inaccessible space' between the hull's skin and the face surface of the sheer clamp L-channel??
     
    Last edited: Aug 5, 2013
  11. waikikin
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    waikikin Senior Member

    Brian,
    What your describing would be a "lap joint, although allowed(to x thickness of plating) is best avoided, with careful fit up most plating members can simply meet edge to edge or T together. It's best to leave behind timber framing conventions in construction of small steel vessels. Plate edges can be temporary longti supported externally with a flat at 90 degrees to hold fair, some 50 x6 with button tacks about at 120-150 & 40-50mm from edge is fine & easily busted off once welding out is complete. Steel is a great material, the glue sets so fast;)
    Jeff.
     
  12. michaeljc
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    michaeljc Senior Member

    Brian – distortion during welding is a whole field in its’ self and way beyond some simple principles that can be described here. Good tradesmen know an awful lot more about the realities of distortion than academics.

    When using relatively light sections as in craft you are designing, never full run weld unless it is essential. Most certainly never full run a fillet weld on one side of a section only unless it is fully stabilised with tacks on the opposite side, . A term tradesmen sometimes used is ‘chain welding’ In the case of a frame member both sides a stitch welded at alternative locations such that entire frame ends up welded. . Usually I would not expect each stitch to be longer than 100mm. Good welds means good penetration which often results in unsightly blemishes on the outside of the hull.

    With that in mind I feel you should experiment with your idea of composite frame. That’s what I do when everyone tells me I am wrong or silly. It would not cost you much. I recall an article somewhere where a NA discusses this approach. I will try to locate it.

    M
     
  13. brian eiland
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    brian eiland Senior Member

    Would you have a little drawing/diagram of what you are describing,...or a reference to a dwg of a common method of making the deck-to-hull joint on a smaller size steel vessel such as a 40 footer? I'm just not quite getting the picture in my head of what you are saying.

    Are you saying that the edge of a steel deck plate would meet the hull skin in a simple T-joint style, and be 'welded out'?

    I would make a little sketch up myself of what I have in mind, but I'm currently over in Thailand with just my laptop....no scanner
     
  14. waikikin
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    waikikin Senior Member

    Hi Brian,
    if you go here...(no direct link to pics?)

    http://www.bruceroberts.com.au/

    & then to "photo gallery".... pick in section 4 "Bermuda 485" then go to page 4 of pics... the flat on edge fairing bar is illustrated, often we would have small angle brackets(these about 40mm & 2 peices tacked "back to back"-the slot is cut so the T of the 2 angles gives stability to the flat member & a nice landing for the clamp) with a slot cut to assist the clamping of the flat onto the plating- also good for squeezing a "rider bar against a inside edge of tranverse framing for attachment.
    The vessel illustrated is typical of "roberts" kit boats, there is plenty of stuff better done with convential framing & level strongback for building.
    Regards from Jeff.
     

  15. brian eiland
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    brian eiland Senior Member

    OK, I went there and I believe I saw what you wanted me to see.

    But correct me if I'm wrong, overall I saw a vessel all 'tacked welded' together. I don't believe they presented the follow-up work of completing all of the welding?? I seem to detect LOTS of areas that if not properly pretreated and coated will be problems down the road with rusting on the interior?

    I went to his photos at the:
    Spray 30: Framework of a S 28, stretched to 30 ft, ready for plating (Steel)
    ...page 4, inside framework.
    I seem to detect that a lot of the 'frames' are not welded to the hull skin?? Are the 'spaces' between these frame edges and the hull skins fully accessible for a proper anti-rusting treatment?
     
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